Figuratively speaking, something that is "hot" is intense.
When applied to people, particularly women, this means, for example, intensely sexy.
When applied to products, it means, intensely popular.
Example 1: "That actress is really hot. Is she single?"
Example 2: "The Prius is really hot right now. You can't just buy it from the dealer and drive it off the lot; you're put on a waiting list and might have to wait months!"